[CliffB]

I've used the Castrol cleaner several times and it cleaned well. I didn't feel as comfortable around paint and other materials as I do with Simple Green.

I clean a lot of customer and my own engines and undercarriages and I prefer Simple Green. I've not had problems with it damaging anything else.

I find the key to either product is letting the stuff work (give it time after it's on) and agitate once it's been on, especially on thick grease and oil buildups. I often let cleaner stay on 30 minutes and more, rewetting with a little spray from time to time and agitating the thick spots again and again.

Best applicator I've found is a 2 plus gallon garden sprayer with a pump and wand. I mix either the Castrol or the Simple Green 50/50 with water unless the job is extra dirty.

IF YOU WANT TO TRY THE ULTIMATE CLEANER...
I found a far more effective cleaner to use when these common ones are not enough. I have used Histo-clear (d-limonene chemically) on trucks and cars for about five years. I get if from the people who make it at National Diagnostics 800-526-3867. A few years ago it was $119.70 for 5 gal plus shipping. I got sold on it after seeing how it cleaned aluminum transmission housings back to like new. It too takes time. It literally eats (converts?) the oils and greases so they wash off.

Histo-clear cleans oil and grease far more effectively than any Castrol, Simple Green or other product I've every tried. A chemist friend / car guy turned me on to it. One of its common uses is cleaning slides used for looking at things in labs under microscopes. It is essentially pure extract (if I understood it right) from oranges. It doesn't pollute as it's 100% natural the way I understand it. Lots of cleaners have some it them. The Histo-clear is just pretty much pure d-limonene. When I need it, it's worth the extra cost for what it does.